Engine starting has a significant impact on engine emissions and driver satisfaction. Inconsistencies in audible engine sound and engine vibration transmitted through the vehicle chassis may reduce customer confidence and satisfaction regarding vehicle operation. In addition, inconsistent air-fuel mixture preparation during a start can produce variation in engine emissions. For example, hydrocarbons may be increased by rich mixtures or by excessively lean mixtures that may cause the engine to misfire.
Conventional mechanically driven valve trains operate the intake and exhaust valves based on the position and profile of lobes on a camshaft. The engine crankshaft is connected to the pistons by connecting rods and to the camshaft by a belt or chain. Therefore, the intake and exhaust valve opening and closing events are based on the crankshaft position. This relationship between the crankshaft position, piston position, and valve opening and closing events determines the stroke of a given cylinder, e.g., for a four stroke engine the intake, compression, power, and exhaust strokes. As a result, engine starting and the first cylinder to fire are determined in part by the camshaft/crankshaft timing relationship and the engine stopping position.
On the other hand, electromechanically driven valve-trains do not have the physical constraints that tie the camshaft and crankshaft together, i.e., there may not be belts or chains linking the camshaft and crankshaft, at least for some valves. Furthermore, full or partial electromechanical valve-trains may not require a camshaft. Consequently, the physical constraints linking the camshaft and crankshafts are broken. As a result, additional flexibility to control valve timing is possible when electromechanical valves are used in an internal combustion engine.
One method to control electromechanical valve operation during an engine start is described in U.S. Pat. No. 5,765,514. This method provides for closing the intake and exhausts valves and then allows the starter to crank the engine. If a signal pulse representing crankshaft rotation through 720 degrees has been generated, an injection sequence for each cylinder and a crankshaft position sequence are set. The injection sequence for the cylinders is initialized when a first crankshaft pulse is generated after generation of a first signal pulse representing crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, suction stroke, compression stroke, and explosion stroke, respectively.
The above-mentioned method relies on a signal that produces a pulse once every 720 degrees at the same engine location. This signal is necessary to begin injection and the subsequent first cylinder combustion event. In other words, the method is dependent on engine stopping position, sensor orientation, and sensor configuration, to produce signals that are necessary to start the engine. Because of this limitation, the method can increase engine noise and vibration depending on which cylinder the sensor configuration causes the engine to start. For example, starting the engine by a cylinder at the end of the engine block may twist the crankshaft more than a cylinder in the center of the engine block, leading to more starting noise and vibration under some conditions.
Further, if the cylinder that follows the 720-degree pulse has a different intake port geometry or injector location, as compared to other cylinders, it may cause the cylinder to breath differently when compared to other cylinders of the same engine. This can produce air-fuel variation and increased emissions.
The inventors herein have recognized these disadvantages of the before-mentioned approach. The inventors have also considered these limitations and determined that the before-mentioned approach simply focuses on operating cylinder valves based on conventional four-stroke engine operation and crankshaft position. With the exception of closing valves before cranking, the method operates intake and exhaust valves during a start similar to a conventional mechanically driven valve system. The method does not recognize that operation of intake and exhaust valves do not have to assume four-stroke timing during an engine start. Therefore, the approach overlooks opportunities to reduce engine emissions, vibration, and noise.
One example approach to overcome at least some of the disadvantages of prior approaches includes a method for starting an internal combustion engine with electromechanically actuated valves. The method comprises: during a first set of operating conditions, performing a first combustion event in a first cylinder of said engine during at least two consecutive starts of said engine; and during a second set of operating conditions, performing a first combustion event in a second cylinder of said engine. This method can be used to reduce the above-mentioned limitations of the prior art approaches.
By repeatedly (e.g., at least during two sequential starts) selecting a cylinder in which to carry out a first combustion event during selected conditions, engine starting and emissions can be improved.
For example, in an engine of “I” configuration, i.e., I4 or I6, selecting the cylinder located closest to the flywheel or near the center of the engine block can reduce torsional vibration created by crankshaft twist during a start. Crankshaft twist is a momentary angular offset between the crankshaft ends that may occur during a start do to engine acceleration. Generally, the first cylinder to fire inducts a high air charge in an effort to accelerate the engine from crank to run speed, thereby, producing a large acceleration. If an engine is started on a cylinder that is furthest from the location of the engine load, i.e., the flywheel, the crankshaft may twist due to the force exerted on the crankshaft by the piston and the distance from the combusting cylinder to the load. Therefore, selecting a cylinder that is located closest to the engine load or that has more support, i.e., a location central to the engine block, can reduce engine vibration during a start under selected conditions. Under other conditions, a different cylinder can be used to start the engine, such as the first cylinder available to carry out combustion.
Also, the above-mentioned method can be used to improve air-fuel control during starting, thereby, reducing emissions. By repeatedly selecting a given cylinder to start an engine under selected conditions, engine emissions can be lowered by reducing air-fuel variation. For example, if a four-cylinder engine is repeatedly started by a first combustion event in cylinder number four, engine emissions can be reduced due to greater predictability. Yet, under other conditions, a different cylinder can be selected. For example, a certain cylinder can be repeatedly used under a first temperature range, and a second cylinder can be repeatedly used under a second temperature range. In this way, the system can adaptively learn the amount of fuel required to start the engine under different conditions while removing variability due to performing first combustion in different cylinders.
The present description thus provides several advantages. Namely, it can reduce crankshaft twist and engine vibration under selected conditions.
And, in an alternative embodiment, by selecting a cylinder based on engine characteristics and delivering unique fuel amounts to each cylinder, air-fuel variation can be reduced.
Note that there are various approaches to identifying engine starting. For example, the engine start can be the period between when an engine begins turning under the power of a starter, until it is rotating at or above a desired idle speed. Another approach is to identify engine starting as the period beginning from key-on until a desired engine speed/air amount is reached.
The above advantages and other advantages and features will be readily apparent from the following detailed description of the embodiments when taken in alone or in connection with the accompanying drawings.